I used to use regular old paint pens-- I can't remember what brand-- and they worked out well... But it's near impossible to find them in singles anymore, now you have to buy them in sets, and if you're like me, you don't need all the colors in the set. So I don't use them much anymore.
As someone else suggested, there are Gundam Markers, which are simply paint pens like I used but with a "Gundam" label on them. Not sure if they're any finer or not, I've never used one. I have a Gundam Marker, but it's part of my collection of Gundam stuff, so it's still in the shrink wrap...
You could always go with a paint brush. There are some insanely fine brushes out there. I've got one that's so fine that I can paint an iris on the eye of a 3 3/4 inch Star Wars action figure. It takes a steady hand and a very little bit of paint, but it works.
Use Gundam Marker the official product, but if your strapped for cash look for Ultra Fine pens and paint markers by Sharpie at your local art supply store and in office supply stores. Also keep some water and a q-tip ready.